Cesium iodide (CsI) is of interest for optical applications because it is the most versatile of all the infrared optical materials, offering transparency from ultraviolet to 60 micrometers wavelengths. However, the poor mechanical properties of single crystal CsI limit its use to relatively small optical components. Significant efforts have been made to improve the mechanical strength of CsI. Introducing divalent impurities such as Mn, Cr, and Ba, into the melt during crystal growth were effective in increasing the strength of single crystal CsI. However, the presence of divalent ions in single crystals has also been shown to be detrimental to transmittance because of the formation in the crystal of defects such as dipoles, precipitates, and aggregates.
Polycrystalline bodies of CsI have been found to have improved mechanical properties as compared to single crystal CsI. One technique to fabricate a polycrystalline body is to introduce subgrains into a single crystal by press forging. Increases in strength were observed in CsI single crystals forged at temperatures between 25.degree. and 200.degree. C. However, the compressive yield strength of the resultant bodies was still relatively low, about 7 MPa. In addition to potential problems such as edge-crack initiation during forging, some researchers have observed that such forged bodies may subsequently undergo primary recrystallization to relieve the strains induced during working, and, of course, recrystallization results in loss of strength in the material.
Another technique to make a polycrystalline body is to hot-press fine powder. However, hot-pressing of alkali halide materials for optical components has generally been avoided because the materials are hygroscopic, making them subject to moisture attack during powder processing. This problem can be alleviated by performing all powder processing and densification operations in a moisture-free environment, such as in a glove box. Polycrystalline CsI that is nine times stronger than single crystal material was fabricated by hot-pressing powder, having and average particle size of about 10 micrometers, at 100.degree. C. for 5 min in a moisture-free atmosphere, However, the transmittance in the extreme long wavelength infrared region (wavelength&gt;15 micrometers) was about 20% less than that of a single crystal as the result of insufficient densification.
Transmittances equivalent to that of single crystal CsI were obtained in bodies hot pressed at a higher temperature of about 150.degree. C. for a longer period of time, 30 min. In this case, however, the fracture was more modest, about two times that of pure single crystal CsI, due to extensive grain growth that occurred during pressing.
Based on previous observations, there is a need to minimize grain growth in the CsI during hot-pressing in order to attain high mechanical strength, while maximizing densification in order to minimize pores which act as optical scattering centers.